1. Field of the Invention
This invention relates to a ball spline so formed that a slider can be moved slidingly on a ball spline shaft, which has longitudinally extending raceway grooves, via a plurality of rolling elements.
2. Description of the Prior Art
Conventionally, a ball spline has been the one in which an outer cylinder having a plurality of balls is moved along a ball spline shaft, and it has been applied to an industrial robot and a transfer machine. Known ball splines include a ball spline having thereon retainers for holding balls, and a ball spline having no retainers.
A ball spline having no retainers is provided as shown in FIG. 19 with a ball spline shaft 3 having axially extending raceway grooves 2 in predetermined portions of an outer circumferential surface thereof, and a hollow outer cylinder having a bore where the ball spline shaft 3 is fitted. The outer cylinder 70 is provided, in an inner circumferential surface of the bore thereof, with raceway grooves 71 which are opposed to the raceway grooves 2 in the ball spline shaft 3, and return passages 72. The ball spline further has spacer rings 74 provided with direction change passages 73, end caps 75, end seals 76 and balls 7 fitted between the opposed raceway grooves. The spacer rings 74, end caps 75 and end seals 76 constitute a slider 78, and are fixed to both ends of the outer cylinder 70 with bolts 77. The width of the window at the edges in cross section of each of the raceway grooves 71 in the outer cylinder 70 is smaller than the diameter of each ball 7 so as to retain the balls 7 (refer to, for example, Japanese Patent Publication No. 61046/1991).
A ball spline having a retainer comprises, as shown in FIG. 20, a ball spline shaft 3 having longitudinally extending raceway grooves 2 in predetermined portions of an outer circumferential surface thereof, and a slider 79 slidable on the ball spline 3. The slider 79 includes an outer cylinder 82 having in the inner surface raceway grooves 80 opposed to those 2 in the ball spline shaft 3, and inclined surface portions 81 for changing the direction of movement of the balls 7; and a retainer 83 for retaining the balls 7 on the inner surface of the outer cylinder 82. The retainer 83 comprises a thin-walled seamless cylinder having a large diameter at the portion which corresponds to the balls 7 in a no-load region of a return passage 84 in the retainer 83, and a small diameter at the portion which corresponds to the balls 7 in a load region of the raceway grooves 80. The retainer 83 is provided with an elongated recess so that the balls 7 rolling in the load region project slightly. The retainer 83 is fixed to the outer cylinder 82 by screwing nut covers 86 on both sides of the inner circumferential surface of the outer cylinder 82 (refer to, for example, Japanese Utility Model Laid-Open No. 52317/1983).
There has been also known a ball spline in which two sliders 88, 88 are provided axially in series on a ball spline shaft as shown in FIGS. 21 and 22. The ball spline shaft 3 is a hollow shaft, which is provided with a pair of raceway grooves 2 formed symmetrically with respect to the axis thereof. Each slider 88 has an outer cylinder 89 provided with race ways opposed to those 2 in the ball spline shaft 3, disc type end caps 90 provided on both axial ends of the outer cylinder 89, and side seals 91 attached to the sides of the end caps 90 which are opposite to the sides in contact with the outer cylinder (casing).
A linear ball-and-roller bearing the construction of which is similar to that of a ball spline has as shown in FIG. 23 a track rail 93 provided with raceway grooves 92 in both of longitudinally extending side surfaces thereof, a casing 95 slidable relatively to the track rail 93, and provided with raceway grooves 94 in the portions which are opposed to the raceway grooves 92, a pair of end caps fixed to both of longitudinal ends of the casing 95, balls 7 rolling circulatingly between the opposed raceway grooves, side seals provided on the outer end surfaces of the end caps, and lower seals so provided as to be opposed to both sides of the rail 93. In the linear ball-and-roller bearing, the raceway grooves 94 in the casing 95 constituting a slider are formed by precision grinding using large-diameter grindstones 96 as shown in FIG. 24, so that the raceway grooves 94 can be formed with a high accuracy.
There is a linear bearing (refer to Japanese Utility Model Laid-Open No. 66057/1975) shown in FIG. 25, the construction of which is intermediate between those of the ball spline shown in FIGS. 19-22 and the linear ball-and-roller bearing shown in FIG. 23. A rail 97 in this linear bearing has a cross-sectional shape very similar to that of the track rail 93 of a linear rolling guide unit. A slider has spline nuts 98 each of which is formed by cutting a portion of an outer cylinder at 120.degree., and a mount 99 placed on the spline nuts 98. Each spline nut 98 is provided in its inner surface with raceway grooves 101 opposed to those 100 in the track rail 97 so that balls 7 roll between the raceway grooves 100, 101. The convex outer circumferential surfaces of the spline nuts 98 are in conformity with the concave inner circumferential surface of the mount 99, and have an automatic aligning performance.
Although the ball spline shown in FIG. 19 is provided with ball retaining portions on side edges of the raceway grooves 71 in the outer cylinder 70, it is very difficult to form raceway grooves 71 having ball-retaining portions. The raceway grooves in this ball spline cannot be formed by using large-diameter grindstones unlike those in the linear ball-and-roller bearing. Consequently, the raceway grooves 71 in the inner surface of the outer cylinder 70 is necessarily formed by broaching, hardening and finishing such as polishing. Therefore, it is difficult to finish the ball spline, and the manday is great. Since the raceway grooves in the inner surface of this outer cylinder 70 are formed by broaching, there is a limit to the axial length of the cylinder with respect to the inner diameter thereof, so that an outer cylinder of a large length cannot be manufactured. Accordingly, a combination of two or more sliders 88, 88 are used as shown in FIG. 21. It is also difficult in the ball spline to form the ball load regions of the raceway grooves 71 in the outer cylinder 70, and direction change passages 73 in the spacer rings 74 and side rings 75 with a high accuracy.
In the ball spline shown in FIG. 20, the raceway grooves 80 in the outer cylinder 82 are provided at both axial end parts of the ball load regions with escape portions formed of inclined grooves for guiding the balls 7 moving in the direction change passages 87 in the retainer 83, so that it is difficult and expensive to form the raceway grooves 80. The outer cylinder 82 to which the retainer 83 is fixed in a conventional ball spline has difficult points in the forming of the raceway grooves therein by broaching and finishing the raceway grooves very precisely. Forming attaching parts, to which the retainer 83 is attached, on the outer cylinder 82 is also troublesome, and the cost of manufacturing the outer cylinder 82 increases.
The linear bearing shown in FIG. 25 involves a problem that the track rail 97 is heavier than that of the ball spline shaft in the ball spline. The spline nuts 98 constituting a slider and formed by cutting a portion of the outer cylinder in the ball spline at 120.degree. does not have a working reference plane, so that it is difficult to form the raceway grooves 101 with a high accuracy.